Ask the Experts
April 14, 2020 - Updated
March 27, 2012 - Originally Posted

White Discoloration After Ultrasonic Cleaning

We are using halogen free solder paste SAC305 type "no clean" for our circuit board assemblies. We are cleaning using an ultrasonic cleaning system with an aqueous cleaning medium specifically designed for ultrasonic cleaners.

The problem is that after cleaning there is a white discoloration on the soldered areas. What could be the cause?


Expert Panel Responses

Without actually seeing the discoloration, it' difficult to tell if it's an oxidation problem possibly caused by a component of the cleaning agent or a true white residue issue. If it is a white residue problem, that may be even more difficult to pinpoint the cause as these may be rooted in the following:
  1. Reaction with the laminate and cleaning agent
  2. Reaction with the laminate and paste
  3. Reaction with the paste and cleaning agent
  4. Reaction involving all three
A few questions:
  • Is the material being rinsed post-cleaning?
  • Is this either a new process for cleaning, a new paste in the process, or a new cleaning agent?
  • What if anything has changed in either the solder or cleaning process (settings, cycle times, temps, etc.)?
  • Was the current entire process evaluated prior to production?
If anything has been recently changed, this may give you an indication of where to look. I would also suggest contacting your cleaning agent vendor for an analysis of your problem and a recommendation.

Pierce Pillon
Laboratory Mgr.
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.

We do not use the soldering materials/process that you do, so I can't add value to your question.

However, I want to comment on the cleaning process you describe.J-STD-001, 8.2.b prohibits ultrasonic cleaning of assemblies unless you have documentation (available for review) that " ...the use of ultrasonics does not damage the mechanical or electrical performance of the product or components being cleaned."

Obviously I don't know your design or the type of parts on the assembly, but I thought it should be pointed out that ultrasonic cleaning can damage quite a wide range of components.

Garry McGuire
Sr. Engineer
NASA/Marshall Space Flight Center
Garry McGuire is a manufacturing process engineer and Chair of the IPC J-STD-001 and IPC/WHMA A-620 Space Addendum committees.

Your residue could be one of two things:
  1. Un-dissolved flux that has been attacked by the chemistry.Partially dissolved flux residues, after drying, can have this white appearance.
  2. Metal salts. The flux forms metal salts as it cleans oxides.After removal of the flux, these salts, which are not water soluble, can be left behind and can be quite difficult to remove. They are not normally harmful.
If spot cleaning with an aggressive solvent such as denatured ethanol does not remove any of the residue, then it is most likely metal salts.Most if not all no-clean residues can be removed with such a solvent. You may want to consult with the paste manufacturer to be certain. Cleaning as soon as possible after reflow will ease removal of the flux residues.

If you find that the residues are residual flux, then you will certainly want to consult with both the paste manufacturer and the cleaning chemistry manufacturer to optimize the process.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Some no clean solder pastes are very difficult to clean. Typically this is due to making a paste pin probable so if this is what you are using, pin probable paste then you can either make a change in the paste or look at a different cleaner. I know Kyzen has developed a cleaner that works for these types of solder pastes. Feel free to contact me if you need specifics.

Good Luck.

Mike Scimeca
FCT Assembly
Mike Scimeca created FCT Assembly after the purchase of Fine Line Stencil, Inc., and consists of two major operations: stencil manufacturing and the manufacturing of electronic assembly products such as solder paste, flux and solder bar.

The white residue is uncleaned no clean flux residue. Ultrasonic cleaning does not clean well for most water soluble fluxes let alone for no clean flux residues. You should call me and we can discuss the risk of residues you are leaving in that no clean flux residue.

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Discoloration can be caused by several things. The first one I would check, is if the discoloration occurs across all solder joints.Is it a film or crystalline by-products of the partially removed flux? If it is a film only, it could be a surface reaction between the flux system and the SAC alloy.

Obviously if you notice crystalline residues usually white to off white in color it is flux residue that have not been removed. Doing ionic tests at this point is important to insure these forms of residues or reactions do not leave conductive residues behind. If ionic molecules are present, cleaning has not been effective.

It is also important to avoid extended times above liquidus during reflow, higher peaks and times above 217C will at times render the residues harder to remove. So lower temperatures, TAL or total area under this curve will make residues after soldering more easily cleaned.

The other concern would be effectiveness of the cleaning agent to remove completely the flux residues and the by-products of soldering. Cycle time, temperature and the concentration of the cleaning agent are important parameters to look at.

In reference to the thin whitish film, this is usually an oxidation reaction between the flux in the solder paste and the solder alloy,it is usually not an issue but mostly cosmetic. Ionic tests should be done to confirm this.

Peter Biocca
Senior Market Development Engineer
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

White residue left on the bump surface may be an indication of over-heat exposure during reflow. As circuit assemblies increase in density, more and smaller components are assembled on to a circuit card. Often times, this requires a longer soak time during the reflow soldering process.

In combination with higher lead-free reflow temperatures,the soils can thermally degrade with the potential to cross link and polymerize. These chemical reactions can render a cleanable solder paste more difficult to clean.

There are a number of other potential factors that may also be the root cause. One is that lead-free flux residues are more difficult to clean. They contain higher molecular weight materials to improve wetting and guard against thermal burn out.

If you have cleaned these residues with the existing process in the past, the issue is most like due to thermal heat exposure at reflow. If not, the issue may be a poorly match cleaning agent for the residue in question. Other factors to investigate are wash time, wash temperature, and bath maintenance.

Mike Bixenman
Kyzen Corp.
Mr. Bixenman is the CTO for Kyzen Corp. Kyzen Corp. is a leading provider of engineered cleaning fluids for high technology manufacturing environments.

I would check the compatibility of the flux to the cleaning solvent. The fluxes can be either rosin or resin based or water soluble based and this has to be defined prior to selecting the cleaning solvents to be used. Using solvents which don't remove all the residues will leave visual traces of the remaining flux films.

Many of the low-solid content fluxes are not made to be cleaned off and when subjected to cleaning solutions some of the residual films are partially removed and hence the light is reflected differently and appear as a whitish residues on the surfaces of the solder joints.

Since this is a paste application the solder joints and the periphery of the solder joints is the only place the flux residues are going to be visible and these most likely will be the areas that turn whitish in appearance.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

After many years of involvement in the PCB and precision cleaning industry the visible white residue issue by far has to be the most common and unpredictable anomaly associated in the fabrication and cleaning steps. Surface residues, such as flux residues, i.e. OA, Rosin or No-Clean and another typically defined as tin-salts which are insoluble in normal solvents,are generally considered a failure condition.

The chemistry and/or composition of the visible white residues are complex; the deposits themselves are of a number of types and origins, and their incidence is associated with both materials and process parameters attributed to before and after cleaning. Fluxing agents react with the tin and lead oxides on the solders' surface.

Doing what flux is designed to do it breaks down the oxides and allows the solder to flow and be joined. The oxides now are defined as metal salts thus effectively inert and water insoluble. Despite their water insolubility, under controlled processing conditions they will be washed away during the cleaning cycle as sufficient quantities of the fluxes soluble ingredients, in many cases the rosin component remains.

Enter the 21'st century and new paste formulas especially those termed"no clean" or low residue is the lower rosin % is now replaced by other active ingredients and thixotropic materials. Plasticizers and other "secret ingredients" sometimes listed on the MSDS. Halogen free solder paste formulas with SAC305 alloys are the most difficult of the new formulas to remove.

It is very important to match the cleaning agent to the flux residue.This is likely a prime factor in why you are seeing the white residue or it can be as simple as not being at the recommended in-use ratio of your cleaning agent. If it cleaned successfully before and now you are seeing white residue verify that the concentration percentage is correct. Also not defined is your wash temperature or time of wash.

These details are also important to confirm. If you wish to discuss this more do not hesitate to contact me.

Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

I can suggest two areas of investigation, it is common for lead-free materials to react with alcohol. Perhaps there is a de minimus long-chain alcohol content in the cleaning material that is causing the chemical reaction with the flux that subsequently cannot be removed by the cleaning fluid.

Two tests suggest themselves:
  1. try cleaning with an alcohol-based cleaner,and see if MORE white residues occur. Alternatively, try cleaning with a material that is completely alcohol-free, and see if the residues occur or can be removed. One such product for testing is MicroCare PowerClean defluxer; available from distributors everywhere.
  2. Another possibility is insufficient rinsing; it may be that due to the temperatures/duration of the soldering process, the cleaning is taking longer and requires more agitation than the current cleaning machine can deliver. A cleaning test using manual cleaning, with a vigorous aerosol rinse, should be sufficient to test this hypothesis.
Overall, the submitter should consider doing a wider variety of tests, with different cleaning systems and chemistries, to try to isolate the failure determine the root cause. This can be VERY time-consuming, but until you do it you're really just shooting in the dark.

Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.

White discoloration means a lot of possibilities... Could be partially cleaned flux residues, or some interaction with joint etc. A photo would help us a lot to identify the issue.I am suspecting the concentration could be low, or the temperature may not beat appropriate level.

Additionally we need to know the wash exposure time and the orientation of the boards inside the ultrasonic tank with respect to location of transducers. If you would like, contact us directly so that we can discuss the details and offer specific recommendations.

Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

A cleaning solvent that is chemically matched to the flux being used is required for satisfactory cleaning. It is also recommended that a cleanliness test of some type is performed as a process control to validate that the cleaning system is meeting the requirements of the contract. If the soldering process for each CCA requires more than an hour to complete, then in-process cleaning should be used pending completion of the soldering and the final cleaning. In your case this is the ultrasonic cleaner (US).

There is nothing wrong with using an ultrasonic cleaning system, as long as you understand its capabilities, and more importantly, its limitations. Be sure you are using a system designed for cleaning of electronics, not mechanical parts. The correct systems for electronics use a random sweeping oscillation to prevent cavitation in the cleaning fluid. Cavitation can totally destroy electronic component wire bonds and die bonds. I recommend Crest have no monetary interest, just a lot of really good experience with them.

Spray-in-air wash systems typically use some type of air knife after the final clean rinse to remove at least most of the rinse water from the CCA. A standard bench-top US system does not provide either a final rinse or a blow-off function. What this means is that the suspended flux solids(and other contaminants) are simply suspended in the wash solvent, be it water,IPA, or some other semi-aqueous solution. If the wash or rinse solution is allowed to simply evaporate, then the reaction products and any suspended solids are simply left behind on the surface of the product, and that is typically the white residue you see.

My recommendation would be to:
  1. Verify the flux-cored wire solder and the solder paste flux match any liquid fluxes that you are using. Never mix flux chemistries.
  2. Talk to the vendor(s) of the flux(s) you are using and get their recommendation on what to use in the US cleaner. CAUTION! Never use flammable solvents or IPA in an ultrasonic cleaner. The vapor produced can be very explosive.
  3. When using a no-clean flux with an ultrasonic cleaner, I have had excellent results using hot DI water (140 deg. F) with a 4 to 6% inert saponifier mix.
  4. Upon completion of the US wash, follow that with a clean hot DI water rinse, and a thorough blow-off, and perhaps a 5-minute bake at 105 deg. C after that.
  5. Perform some type of periodic cleanliness test. 6. Perform some type of magnified inspection to make sure the residues don't come back.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Reader Comment
Is it residue? Take a solder sample from the same lot (do not print)and run through oven and then place in ultrasonic cleaner for similar time as your problem board(s).

"Residue" present? If yes, look at condition of LPI mask cure. No? Print with paste, reflow and place in cleaner. Residue? Review what appears to be a solvent for the residue, do ionic contaminant testing, and investigate compatibility of cleaner chemistry with paste as recommended by others.
Robb Spoerri, USA

More than likely the Waxes as the Rosins will be normally very soluble portion of flux. The Paste manufacturers use Waxes for tack life and they are extremely difficult to remove if left on any longer than 48 hours after reflow. The Wax is insoluble in most normal Machine chemistries once aged any longer than 48 hours but can be cleaned using hand aerosol cleaners/defluxers due to the more active solvents used in aerosols.

For best results get them cleaned in the tank at around 60C the hotter the better, with a higher Ph solution or the US or FA+ works well from Zestron in Ultrasonics if done quickly.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
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